Photopolymerizable inkjet inks using (meth)acrylic acid esters and photopolymerizable inkjet inks using a combination of (meth)acrylic acid esters and vinyl ethers have been widely known (see PTL 1 etc.).
However, many of monomers used in conventional photopolymerizable inkjet inks are toxic. Especially, most of (meth)acrylic acid esters, which are readily available with low cost, have high toxicity in terms of skin sensitivity, which causes allergy reactions with skin upon contact with them. Conventional art has not provided any solution to this problem.